Well treating process and composition

ABSTRACT

A process for treating a subterranean zone by emplacing therein a hardenable aqueous slurry and then permitting the slurry to harden, where the slurry comprises a hydraulic cement, water, sodium bentonite, sodium metasilicate, and a hydroxyethyl cellulose. 
     The composition and process employing same is particularly useful in the treatment of oil and gas wells where cementing of a weak formation or very long string cementing, in a single stage, is desired.

CROSS-REFERENCE TO RELATED APPLICATION

This application is a divisional of copending application Ser. No.461,423 which was filed on Jan. 27, 1983.

BACKGROUND OF THE INVENTION

The invention pertains to a process for treating subterranean zones,particularly oil and gas wells where incompetent or weak formations areencountered, with lightweight, aqueous hydraulic cement slurry and acomposition useful in this process. In cementing particularly longstrings of pipe in a wellbore, i.e., one or two thousand feet or more,it is often necessary to perform the cement job in multiple stagesbecause the excessive weight of the long column of cement slurry wouldfracture the subterranean formation.

Cement slurries comprising hydraulic cements, bentonite and largeamounts of water have been employed in well cementing where alightweight slurry is desired. Sodium metasilicate has also beenemployed in cement slurries in well cementing. Cellulose derivates havelikewise been employed in well cement slurries in the past.

In order to cement long strings of pipe, oil and gas producers havepreviously resorted to multiple-stage cement jobs with resultantextended lost rig time or have used more expensive additives such asglass or ceramic microbubbles to lighten cementing slurries. Previouswater-extended lightweight slurries employing bentonite or sodiummetasilicate have generally suffered from poor performance due toexcessive free water content or lack of reasonable setting time orultimate strength.

SUMMARY OF THE INVENTION

A process for treating subterranean zone by emplacing therein ahardenable, aqueous slurry and thereafter permitting said slurry toharden, where the slurry comprises a hydraulic cement, and based on 100parts of said cement by weight:

A. about 150 to about 250 parts water;

B. about 10 to about 15 parts sodium bentonite;

C. about 0.5 to about 1.5 part sodium metasilicate;

D. about 0.5 to about 1.5 part of a hydroxyethyl cellulose.

While the individual components have previously been used in well cementapplications, it is critical that the components mentioned above all bepresent within the designated ranges or a slurry having acceptableproperties for "long-string" cementing applications will not beobtained. Such a slurry preferrably has a weight of between about 10 andabout 12 pounds per gallon in order to form an acceptable fluid for thetreatment in weaker subterranean formations or in applications where along column of cement slurry will be emplaced in a single operation.Utilizing the process and slurry of the invention, long-string cementingjobs may be performed and circulation of cement slurry in the annulusbetween the tubing and the wall of the borehole may be obtained, evenback to the surface from a depth of several thousand feet.

Preferably, an aqueous slurry is employed having a weight between about10.5 and about 11.5 pounds per gallon and preferably a free watercontent, as determined by American Petroleum Institute (API) standards,of less than about 10 ml.

DETAILED DESCRIPTION OF THE INVENTION

The process of the invention is suitably performed with standard oilfield equipment used for pumping cement slurries into boreholes ofsubterranean formations. The lightweight cement slurry of the instantinvention is also prepared using standard oilfield cement blending andmixing equipment by first combining all dry additives with the finelydivided hydraulic cement component, combining any liquid additives withthe mix water to be employed and then mixing the dry components with theso blended liquid components to obtain the slurry.

The hydraulic cement component is a cementitious material whichtypically is a mineral material which when wetted with water changesfrom a finely or coarsely divided material into a continuous hardenedmaterial. Portland cements, alumina cements and pozzolan cements arerepresentative. Portland cement is preferred for utilization in thepresent invention particularly those Portland cements known as API classG and class H cements. Unless otherwise specified, all quantitiesrelated to the composition of the resulting cement slurry are in partsby weight and based upon 100 parts of the hydraulic cement component.

In formulating the cement slurry of the invention, about 150 to about250 parts water will be added to the hydraulic cement. Preferably, anamount of water sufficient to yield a resultant aqueous slurry having aweight of between about 10 and about 12, more preferably between about10.5 and about 11.5 pounds per gallon is employed. More preferably,about 180 to about 220 parts water are employed per 100 parts of thehydraulic cement.

Component B of the invention slurry, which is utilized in amounts ofbetween about 10 and about 15 parts and preferably between about 11 andabout 14 parts per 100 parts hydraulic cement, is a finely divided,water-swellable material known as bentonite. It is comprised primarilyof the clay montmorillonite. Since it is desired that the bentonite takeup and hold a large quantity of the water in the cement slurry, it ispreferably used mainly in the sodium form. This form has a high swellingcapacity in water while the calcium form of bentonite does not swellappreciably in water. The sodium form of bentonite is generally known asWyoming or Western bentonite. It is commonly used in the oilfield, andpreferably meets API specification 10 requirements.

Component C of the invention slurry, employed in amounts from about 0.5to about 1.5 part per 100 parts cement is sodium metasilicate. Sodiummetasilicate is available both in the anhydrous and pentahydrate forms.Since the anhydrous form has a higher bulk density and is therefore moreeconomical to handle, it is preferred for use in the present invention.The amounts of sodium metasilicate expressed herein refer to theanhydrous basis and where the pentahydrate form is utilized, acorrespondingly greater amount would be employed to account for thelower bulk density thereof. Preferably, about 0.75 to about 1.25 part isemployed and most preferably about 1 part is employed per 100 partscement in the invention slurry.

Component D, in the amount of about 0.5 to about 1.5 part, is ahydroxyethyl cellulose. Numerous hydroxyethyl cellulose products areavailable on the market having varying degrees of molar substitution andavailable in different molecular weights. The term hydroxyethylcellulose, as employed herein, is also meant to include thosehydroxyethyl celluloses which bear methoxy substituents on at least someof the active hydroxyl sites of the cellulose molecule and which arecommonly known as hydroxyethyl methylcellulose or HEMC. Since theabsolute molecular weights of such hydroxyethyl celluloses are difficultto determine, relative molecular weights are generally expressed interms of the viscosity generated at a given temperature for an arbitraryconcentration of the polymer in an aqueous solution. Preferred for usein the instant invention are hydroxyethyl celluloses having about 2.5moler substitution. Also preferred in the invention are hydroxyethylcelluloses which bear no methoxy substituents and which exhibit, at 25°C., a Brookfield viscosity of between about 25 and about 400 centipoise(cps) at a 2% concentration in fresh water.

The amount of Component D employed in the instant slurries is preferablybetween about 0.75 and about 1.25 part, more preferably about 1 part per100 parts cement. However, when the higher molecular weight hydroxyethylcelluloses are to be employed in the instant invention it may bedesirable to utilize less than it would be if the hydroxyethyl cellulosewere of lower molecular weight. Accordingly, if the amount of ahydroxyethyl cellulose to be employed in the instant invention shouldrender the resulting slurry thicker than is desirable for easy pumpingunder downhole conditions, then the amount of the hydroxethyl cellulosemay be decreased. Usually the amount of sodium metasilicate in ComponentC may then be correspondingly increased in order to avoid a slurryhaving excess free water. Conversely, where the slurry exhibits very lowfree water content but is thicker than desired, the amount of sodiumsilicate may be reduced and the amount of the hydroxyethyl celluloseemployed may be increased.

In addition to the foregoing required components, other standard oilwell cement slurry additives may be included. It is preferable, however,to avoid addition of organic dispersants such as sulfonated polyaromaticcompounds and sulfonated lignins since these tend to destroy the waterretention capability of the required components of the slurry. However,other standard oilfield cement additives such as accelerators, retardersand extenders such as gilsonite or pulverized coal, silica flour orsilica powder (e.g., diatomaceous earth) may optionally be included inthe cement slurry.

In one preferred mode of the invention, up to about 10% sodium chloridemay be incorporated in the mix water, based upon the weight of water.Preferably, from about 1 to 5% and more preferably about 3% sodiumchloride, is included in the water to yield a "salt water" cementsystems.

In another preferred embodiment of the invention, up to about 15 parts,based upon 100 parts cement, of gilsonite, ground coal, silica flour, ordiatomaceous earth is incorporated in the cement slurry. In anotherpreferred embodiment of the invention, the hydraulic cement component iscomprised of a 50:50 mixture of Portland cement and a pozzolanic flyashmaterial. Most preferably, the aqueous slurries of the invention exhibitan API free water content of about 7 ml or less, more preferably about3.5 ml or less and most preferably about 2 ml or less.

SPECIFIC EMBODIMENTS OF THE INVENTION Example 1

A cement slurry is prepared by combining 100 parts Class G cement withabout 12 parts sodium bentonite, 1 part sodium metasilicate and 0.75part hydroxyethyl cellulose having a molar substitution of about 2.5 andexhibiting a Brookfield viscosity of between about 200 to 400 centipoiseat 25° C. as a 2% aqueous solution. This dry mixture of components isthoroughly blended and then added to about 210 parts fresh water withvigorous mixing. Upon testing the resultant slurry according to APIspecifications for free water, 0 ml of free water is observed. When thisslurry is tested according to API specifications after 72 hours at 88°C., a compressive strength of 320 psi is observed.

Example 2

The slurry of Example 1 is prepared but to the 210 parts water is added3%, based on weight of water, sodium chloride. This slurry exhibits 8 mlfree water and has a compressive strength of about 500 psi after 72hours at 88° C.

Example 3

The slurry of Example 2 is again prepared except that the amount of thehydroxyethyl cellulose component is raised from 0.75 part to 1 part. Theslurry exhibits a free water content of 3 ml.

Example 4

To 100 parts of a 50:50 mixture of Class G cement and flyash is added 12parts sodium bentonite, 1 part sodium silicate, and 1 part of thehydroxyethyl cellulose mentioned in the preceding examples. Anadditional 10 parts of acid washed diatomaceus earth, a finely powderedsilica, is added to the dry mixture. This is blended with about 210parts water and gives a compressive strength nearly twice that of theslurry of Example 1 in only 24 hours rather than 72 hours.

Example 5

When any one of the sodium bentonite, the sodium silicate, or thehydroxyethyl cellulose is omitted from the formulation of Example 1, anAPI free water of 10 ml or more is noted in the resulting slurries.

Example 6

A slurry is prepared in the fashion of Example 1 except that about 1part of a hydroxyethyl methylcellulose (HEMC) is substituted for thehydroxyethyl cellulose of that example. The hydroxyethyl methylcellulosehas about 0.2 methoxyl degree of substitution and about 2.4 hydroxyethylmolar substitution. A 2% aqueous solution of the HEMC at about 25° C.has a Brookfield viscosity between about 600 and 725 centipoise (60 and30 RPM). The resulting slurry exhibits about 2 ml API free watercontent.

Example 7

A slurry of the invention formulated as described in Example 1 is pumpeddown 12,000 feet of tubing in a borehole and up the annulus formedbetween this tubing and the face of the borehole until cement returnsare seen at the surface. In this fashion, a long casing cementing job isperformed in a single stage where it ordinarily requires the addedexpense of multiple stages with conventional slurries. A bond log isperformed on casing cemented with the aforementioned slurry and showsgood to excellent bonding. Bond logs from cement jobs performed in thesame area using other available lightweight systems show poor cementbonding.

What is claimed is:
 1. An aqueous hydraulic cement slurry adapted foremplacement in subterranean formations and being relatively light inweight which comprises a hydraulic cement and, based on 100 parts byweight of said cement:A. about 150 to about 250 parts water; B. about 10to about 15 parts sodium bentonite; C. about 0.5 to about 1.5 partsodium metasilicate; D. about 0.5 to about 1.5 part of a hydroxyethylcellulose.
 2. An aqueous hydraulic well cement slurry which comprises ahydraulic cement and, based on 100 parts of said cement:A. about 210parts water; B. about 12 parts sodium bentonite; C. about 1 part sodiummetasilicate; D. about 1 part of a hydroxyethyl cellulose; wherein thehydraulic cement comprises a Portland cement.
 3. The slurry of claim 2wherein the hydraulic cement further comprises a pozzolan.
 4. The slurryof claim 1 wherein the aqueous slurry has a weight of between about 10.5and about 11.5 pounds per gallon.
 5. The slurry of claim 1 wherein theaqueous slurry has an API free water content of about 7 ml or less. 6.The slurry of claim 1 wherein Component (D) is a hydroxyethyl cellulosehaving a molar substitution of about 2.5.
 7. The slurry of claim 1wherein the hydraulic cement comprises Portland cement.
 8. The slurry ofclaim 7 wherein the hydraulic cement further comprises a pozzolan. 9.The slurry of claim 7 wherein the hydraulic cement comprises about a50:50 blend, by weight, of Portland cement and flyash.
 10. The slurry ofclaim 1 wherein about 1 to about 5 percent sodium chloride is present inthe slurry, based on the weight of water present.
 11. The slurry ofclaim 1 wherein about 180 to about 220 parts water are present.
 12. Theslurry of claim 11 wherein about 1 part Component (C) and about 1 partComponent (D) are present.
 13. The slurry of claim 12 wherein about 1 toabout 5 percent sodium chloride is present in the slurry, based on theweight of water present.
 14. The slurry of claim 13 wherein thehydraulic cement comprises Portland cement.
 15. The slurry of claim 14wherein the hydraulic cement further comprises a pozzolan.
 16. Theslurry of claim 15 wherein the hydraulic cement comprises about a 50:50blend, by weight, of Portland cement and flyash.
 17. The slurry of claim1 wherein the aqueous slurry has a weight of between about 10 and about12 pounds per gallon and an API free water content of about 7 ml orless, the hydraulic cement comprises about a 50:50 blend, by weight, ofPortland cement and a pozzolan and the aqueous slurry is emplaced in thesubterranean zone by pumping it down tubing situated in a borehole intosaid zone and into an annulus formed between the outside of said tubingand the face of said borehole, at least until some of the slurry isreturned to surface from the zone, and thereafter permitting theemplaced slurry to harden.
 18. The slurry of claim 1 wherein thehydraulic cement comprises Portland cement; about 180 to about 220 partswater; about 10 to about 15 parts sodium bentonite; about 0.75 to about1.25 part sodium metasilicate and about 0.75 to about 1.25 part of ahydroxyethyl cellulose are present in the slurry.